Billionaires like to declare no income, which is why they pay such low taxes. But they spend like kings, not like people who have no income. But you can't spend unrealised gains. So they either realise their gains and pay taxes, or get their spending money elsewhere. Since they spend a lot of money, and "elsewhere" wants its money back eventually, it sounds like "elsewhere" has to be a bank.
No. The most plausible explanation is they sold stock and paid capital gains. They do it all the time. Don't believe me? Just look at the insider transaction reports that all public companies file. e.g Bezos sold 1.2 billion last month: https://finance.yahoo.com/quote/AMZN/insider-transactions/
It seems like the most plausible explanation.